1. Field of the Invention
The subject invention relates to methods and apparatus for containing radioactive sources and, more particularly, to containing low level radioactive sources for shipment and subsequent handling.
2. Description of the Invention Background
In various industrial and commercial operations, there is a need to measure the density or specific gravity of process liquids, suspensions or bulk materials flowing in pipes or contained in tanks or vessels. There is also the need to detect the presence or absence of process material in tanks, pipes, hoppers and chutes, as well as to continuously monitor the level of material therein.
A prior technique of measuring density and/or level within tanks, pipes, hoppers and chutes (collectively referred herein as "conduits") employed a technique known as radiation absorption gauging. Another application of the use of radiation absorption gauging techniques is in measuring the level of molten metal in a continuous casting apparatus as shown in Cahill et al., U.S. Pat. No. 5,564,487, the disclosure of which is hereby incorporated by reference. The radiation absorption gauging technique includes the provision of a radioactive source and a detector spaced therefrom. The radioactive source comprises a material that is continuously disintegrating and which emits particles and energy in the form of alpha, beta and gamma rays in transitioning to a lighter elemental material. The detector is responsive to the impingement of these particles/energy to provide a given signal level which is inversely proportional to the square of the distance between the source and the detector. When an object is disposed between the source and detector, or the aggregate density of medium between the source and detector is otherwise increased, the source emissions will be absorbed by the increased mass of the intervening medium and the output signal from the detector will be proportionately reduced.
This principle may be used to detect the density of material within a conduit by disposing a radioactive source on one side of, or within, the conduit and a detector on the opposite side. As such, the radiation field created by the source passes through the conduit and the material therein before impinging on the detector. By proper calibration techniques, the material passing through the conduit absorbs radiation thereby diminishing the output signal from the detector by a measurable amount.
The same principles are involved in gauging the level of material in a vessel. In this situation, the radioactive source is disposed on one side of a conduit and the detector is disposed on the opposite side of the conduit. There is thus created a radiation field that spans a portion of the height of the conduit. The intensity of the radiation impinging on the detector and the output signal therefrom is inversely proportional to the degree to which the material in the conduit absorbs radiation which, in turn, is a function of the level of the material in the conduit.
In each of the above described measuring techniques, the radioactive source has historically comprised material such as cesium-137 which emits radiation in the range of 0.001 to over 5 curies. Radioactive sources of such intensity are regulated by federal and state regulations throughout the world due to concerns associated with radiation sources of such levels. Such concerns include those associated with the handling of such radioactive sources to insure that workers were not exposed to dangerously high levels of radiation. Similarly, the shipment of such radiation sources in interstate commerce has been heavily regulated to ensure the safe and error free handling of radioactive sources. Further, the disposal of radiation sources of such intensity is regulated to insure that radiation sources do not contaminate landfills, water supplies or other natural resources.
In response to the several concerns relating the use of strongly radioactive sources, the applicant and others have developed technology whereby lower level radioactive sources may be employed. Such sources have radioactive material, such as cesium-137, colbalt-60 or americium-241 in units measured in the micro-curie (10.sup.-6 curie) range. For example, a cesium-137 radioactive source may be provided in units of less than 10 micro-curies substantially exempt from regulation. By means of comparison, common household smoke detectors may contain americium radioactive sources which contain radioactive sources in the range of 1 micro-curie. Such low level radioactive sources are usually not regulated to nearly the extent as stronger radioactive sources. For present purposes, the regulation of concern is that radioactive sources not be shipped or installed in packages containing more than 100 micro-curies of radioactive material. Further, some of the shipping requirements of low level radioactive sources require that less than a predetermined amount, such as 0.5 milliRem per hour, be emitted at the surface of the container. Indeed, some jurisdictions have imposed standards at the level of 0.1 milliRem per hour. However, even low-level radioactive sources mounted on small disks, spheres, pellets, rods, etc. are shipped loose in containers and require further handling and assembly into the radiation absorption gauging system on site. Heretofore, there has not been a preassembled package of low-level radioactive sources for immediate insertion into a measuring system. Furthermore, there has not been a suitable, reusable shipping container for a plurality of low-level radioactive sources.
Thus, there is a need for a preassembled holder for a plurality of low level radioactive sources. There is also a need for a shipping container for low level radioactive sources which will allow for the safe shipment of such sources without regulation. There is still another need for an assembly of low-level radioactive sources which may be repeatedly used or taken off-line.
The instant invention is directed toward a holder and a container for low level adioactive material which can be used in connection with density, level and/or belt weighing gauge systems for radiation absorption gauging.